Knowledge regarding the fundamental mechanisms underlying active transport of bile salts and vitamin B-12 across the brush border plasma membrane of ileal epithelial cells is of crucial importance in understanding intestinal absorption of these physiologically and nutritionally important substances. The goal of this investigation is describe the molecular events, elucidate the energy sources and identify the membrane constituents which are involved in transfer of bile salts and vitamin B-12 from the lumen of the ileum into the enterocytes. For the purpose we will prepare membrane vesicles derived from intestinal epithelial cells and determine the characteristics of uptake of bile salts and vitamin B-12 by these vesicles. The effects of ions, ion gradients, adenosine triphosphate, temperature, pH, sulfhydryl reagents, competitive substrates and membrane potential upon vesicular binding and transport of bile salts and vitamin B-12 will be ascertained. We will also solubilize and purify the bile salts and vitamin B-12 transport systems and incorporate them into phospholipid liposomes. The reconstituted transport system will be utilized to identify the membrane constituents which are responsible for transmembrane transfer to bile salts and vitamin B-12 and to determine how these transport systems interact with other components of the brush border membrane. The information obtained in this investigation will provide a fuller understanding of the functional principals governing intestinal absorption of bile salts and vitamin B-12 and should also contribute significantly to a broader understanding of transport physiology and pathophysiology in general.